Group III-nitride layers with patterned surfaces

ABSTRACT

A fabrication method produces a mechanically patterned layer of group III-nitride. The method includes providing a crystalline substrate and forming a first layer of a first group III-nitride on a planar surface of the substrate. The first layer has a single polarity and also has a pattern of holes or trenches that expose a portion of the substrate. The method includes then, epitaxially growing a second layer of a second group III-nitride over the first layer and the exposed portion of substrate. The first and second group III-nitrides have different alloy compositions. The method also includes subjecting the second layer to an aqueous solution of base to mechanically pattern the second layer.

BACKGROUND

1. Field of the Invention

The invention relates to electrical and optical devices that incorporatecrystalline group III-nitrides.

2. Discussion of the Related Art

Crystalline group III-nitride semiconductors are used in both electricaldevices and optical devices.

With respect to electrical devices, group III-nitrides have been used tomake field-emitters. A field-emitter is a conductive structure with asharp tip. The sharp tip produces a high electric field in response tobeing charged. The high electric field causes electron emission from thetip. For this reason, an array of field emitters can operate a phosphorimage screen.

One prior art method has fabricated arrays of field-emitters from groupIII-nitrides. Group III-nitrides have chemical and mechanical stabilitydue to the stability of the group III atom-nitrogen bond. Such stabilityis very desirable in devices that use an array of field-emitters.

The prior art method grows the field emitters from group III-nitrides.The growth method includes epitaxially growing a gallium nitride (GaN)layer on a sapphire substrate, forming a SiO₂ mask on the GaN layer, andepitaxially growing pyramidal GaN field-emitters in circular windows ofthe mask. While the growth method produces field-emitters of uniformsize, the field emitters do not have very sharp tips. Sharper tips aredesirable to produce higher electron emission rates and lower turn-onvoltages.

With respect to optical devices, group III-nitrides have high refractiveindices. Materials with high refractive indices are desirable in themanufacture of photonic bandgap structures. For a fixed photonicbandgap, such materials enable making a photonic bandgap structure withlarger feature dimensions than would be possible if the structure wasmade from a lower refractive index material.

One method for making a planar photonic bandgap structure involves dryetching a smooth layer of group III-nitride. Unfortunately, the chemicalstability of group III-nitrides causes dry etchants to have a lowselectivity for the group III-nitride over mask material. For thatreason, a dry etch does not produce a deep surface relief in a layer ofgroup III-nitride. Consequently, the dry-etch method only produces thinplanar photonic bandgap structures from group III-nitrides.

Unfortunately, light does not efficiently edge couple to thin planarstructures. For this reason, it is desirable to have a method capable offabricating a photonic bandgap structure with a higher surface relieffrom a group III-nitride.

BRIEF SUMMARY

Herein a mechanically patterned surface has an array of deformationstherein, e.g., an array of holes, trenches, or physically rough regions.

Various embodiments provide methods for fabricating layers of groupIII-nitride with mechanically patterned surfaces. The patterned surfacesprovide functionalities to the resulting structures. The fabricationmethods exploit the susceptibility of nitrogen-polar (N-polar) groupIII-nitride layers to attack by strong bases. The methods use basicsolutions to wet etch a layer of group III-nitride in a manner thatproduces a patterned surface. Exemplary patterned surfaces providephotonic bandgap structures and field-emitter arrays.

In a first aspect, the invention features a fabrication method. Themethod includes providing a crystalline substrate and forming a firstlayer of a first group III-nitride on a planar surface of the substrate.The first layer has a single polarity and also has a pattern of holes ortrenches that expose a portion of the substrate. The method includesepitaxially growing a second layer of a second group III-nitride overboth the first layer and the exposed portion of substrate. The first andsecond group III-nitrides have different alloy compositions. The methodincludes subjecting the second layer to an aqueous solution of base tomechanically pattern the second layer.

In a second aspect, the invention features an apparatus with amechanically patterned surface. The apparatus includes a crystallinesubstrate with a planar surface and a plurality of pyramidalfield-emitters located over a portion of the surface. The apparatusincludes a layer of a first group-III nitride, which is located onanother portion of the surface, and a layer of a second groupIII-nitride, which is located over the layer of the first groupIII-nitride. The layer of the second group III-nitride is free ofpyramidal surface structures. The field-emitters include the secondgroup III-nitride. The first and second group III-nitrides havedifferent alloy compositions.

In a third aspect, the invention features an apparatus that includes acrystalline substrate and a mechanically patterned layer of a firstgroup III-nitride that is located on a planar surface of the substrate.The apparatus also includes a layer of a second group III-nitride thatis located on the mechanically patterned layer of the first groupIII-nitride. The layer of second group III-nitride has a pattern ofcolumnar holes or trenches therein. The first and second groupIII-nitrides have different alloy compositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a planar structure for afield-emitter array of group III-nitride;

FIG. 1B is a cross-sectional view of another planar structure for afield-emitter array of group III-nitride field-emitters;

FIG. 1C shows a flat panel image display that incorporates thefield-emission array of FIG. 1A or 1B;

FIG. 2 is a cross-sectional view of a structure that incorporates alayer of group III-nitride that is mechanically periodically patternedwith holes or trenches;

FIG. 2A is top view of one embodiment of photonic bandgap device thatincorporates a structure represented by the structure of FIG. 2;

FIG. 2B is top view of another embodiment of a photonic bandgap devicethat incorporates a structure represented by the structure of FIG. 2;

FIG. 2C is top view of another embodiment of a photonic bandgap devicethat incorporates a structure represented by the structure of FIG. 2;

FIG. 3 is a flow chart illustrating a method for fabricating structureswith patterned layers of group III-nitride as shown in FIGS. 1A-1B, 2,2A, and 2B;

FIG. 4 is oblique view scanning electron micrograph (SEM) of a structuremade by an embodiment of the method of FIG. 3 in which the wet etch timeis short;

FIG. 5 is top view SEM of a structure made by an embodiment of themethod of FIG. 3 in which the wet etch time is of intermediate length;

FIG. 6 is a top view SEM of a structure made by an embodiment of themethod of FIG. 3 in which the wet etch time is long; and

FIG. 7 is a flow chart for specific method of fabricating GaN structureswith patterned layers as shown in FIGS. 1A-1B, 2, 2A, and 2B.

In the Figures and text like reference numbers refer to similarelements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The chemical stability of the bond between group III metals and nitrogencauses group III-nitride semiconductors to be chemically resistant tomany etchants. Nevertheless, aqueous solutions of strong bases will etcha nitrogen-polar surface of a layer of group III-nitride. Such wetetchants are able to mechanically pattern a group III-nitride layer thatis already been polarity patterned. Exemplary patterned surfaces producefield-emitter arrays, as shown in FIGS. 1A-1B, and photonic bandgapstructures, as shown in FIGS. 2, 2A, and 2B.

FIG. 1A shows a field-emitter array 10A. The field emitter array 10Aincludes a substrate 12 and a regular pattern of laterallyinter-dispersed columnar first and second regions 14, 15. The first andsecond regions 14, 15 cover a planar surface 17 of the substrate 12. Thesubstrate 12 is a crystalline material such as silicon carbide (SiC) or(0 0 01)-plane sapphire. The first and second regions 15, 16 includerespective (0 0 0 1)-polarity and (0 0 0 {overscore (1)})-polarity formsof a group III-nitride. The group III-nitride of the regions 14, 15 hasa wide lattice mismatch with the planar surface 17 of the substrate 12.

Herein, the (0 0 0 {overscore (1)})-polarity and (0 0 0 1)-polarityforms of a group III-nitrides are referred to as the N-polar andmetal-polar forms. The N-polar and metal-polar forms have oppositeintrinsic polarizations. The free surfaces of flat N-polar andmetal-polar layers terminate with layers of nitrogen atoms and groupIII-metal atoms, respectively.

The first and second columnar regions 14, 15 have physically differentsurfaces and thus, form a mechanically patterned layer of groupIII-nitride on the substrate 12. The first regions 14 include one ormore hexagonal pyramids 16 of the group III-nitride. Thus, the firstregions 14 have non-flat exposed surfaces, which include sharp tips 20.The second regions 15 include smooth layers of the group III-nitride andare devoid of pyramidal structures. Thus, exposed surfaces 22 of thesecond regions 15 are smooth and flat. The surfaces 22 of the secondregions 15 are also farther above the planar surface 17 of the substrate12 than the highest tips 20 in the first regions 14.

The hexagonal pyramids 16 of the first regions 14 have sharp apicalpoints 20 and thus, can function as field emitters. The apical tips 20have diameters of less than 100 nanometers (nm). In embodiments wherethe group III-nitride is GaN, the pyramids 16 have tips 20 withdiameters of less than about 20 nm-30 nm and have faces that make anglesof about 56°-58° with the planar surface 17. The pyramids 16 have sixfaces that are (1 0 {overscore (1)}{overscore (1)}) facets.

Within a single first region 14, the distribution of the hexagonalpyramids 16 and apical tips 20 is random. Various first regions 14 mayhave different numbers of the hexagonal pyramids 16. The sizes of thefirst regions 14 are constant, because the inter-dispersed secondregions 15 have the same size and regular lateral distribution.

In the second regions 15, the layer of first group III-nitride rests ona much thinner base layer 18 that is made of a second group III-nitride.The second group III-nitride has a wide lattice mismatch with the planarsurface 17 of the substrate 12. More importantly, the second groupIII-nitride grows with metal-polarity on the surface 17 of the substrate12.

FIG. 1B shows an alternate embodiment of a field-emitter array 10B. Thefield emitter array 10B includes a substrate 12 and a regular pattern ofinter-dispersed columnar first and second regions 14, 15 as alreadydescribed with respect to FIG. 1A. In the field-emitter array 10B, thesecond regions 14 include overlapping hexagonal pyramids 16 rather thanisolated pyramids 16 as in the field-emitter array 10A. Also, thepyramids 16 have a range of sizes and rest on a thick layer 19 ofN-polar group III-nitride rather than directly on the planar surface 17as in the field-emitter array 10A of FIG. 1A. In the field-emitter array10B, the hexagonal pyramids 16 still have sharp apical points 20 andthus, can still function effectively as field-emitters. The apical tips20 are still lower than the exposed top surfaces 22 of the secondregions 15.

FIG. 1C shows an embodiment of a flat panel image display 24. Thedisplay 24 incorporates a field-emitter array 10, e.g., array 10A or 10Bfrom FIGS. 1A and 1B. The display 24 also includes metallic electrodes26 and a phosphor screen 28. The metallic electrodes 26 are supported bythe flat top surface 22 of the field emitter array's second regions 15.The top surfaces 22 support the metallic electrodes 26 along a planethat is nearer to the phosphor screen 28 than are the tips 20themselves. For that reason, the metallic electrodes 26 are able tocontrol emission of electrons from the field-emitter array. The metallicelectrodes 26 function as control gates for field-emitters in adjacentfirst regions 14. Supporting the metallic electrodes 26 on the secondregions 22 conveniently avoids a need to self-align the electrodes onindividual tips 20. Such an alignment process would be complex, becausethe positions of the tips 20 are random in individual first regions 14.

FIG. 2 shows another structure 30 that has a mechanically patternedlayer 32 of a first group III-nitride. The layer 32 is located on aplanar surface 17 of crystalline substrate 12 e.g., the (0 0 01)-planeof a sapphire substrate. The layer 32 includes a regular array ofidentical columnar holes or trenches 34. The holes or trenches 34 havesubstantially rectangular cross sections and traverse the entirethickness of layer 32. The layer 32 rests on a mechanically patternedbase layer 18. The base layer 18 is a second crystalline groupIII-nitride with a different alloy composition than the first groupIII-nitride. The base layer 18 aligns epitaxially on the planar surface17 to be group III metal-polar.

For the pair of layers 18 and 32, exemplary pairs of second and firstgroup III-nitride semiconductors are: the pair AlN and GaN or the pairAlN and AlGaN.

The layer 32 has thickness that is typically 100-10,000 times than thethickness of the base layer 18. An exemplary GaN layer 32 has athickness of 30 μm or more, and an exemplary AlN base layer 18 has athickness of only about 20 nm-30 nm. The base layer 18 only has to bethick enough to align the polarization of another layer located on thebase layer 18.

In optical devices, the layer 32 usually functions as an optical core ofa planar waveguide. The waveguide receives input light 36 via an edge 37and transmits output light 38 via an opposite edge 39. Such edgecoupling of the layer 32 to optical fibers and other optical waveguidesis more efficient for embodiments in which the layer 32 is thicker. Itis thus, advantageous that the layer 32 can be relatively thick, i.e.,30 μm or more, because such a thicker layer 32 enables efficient endcoupling to standard optical fibers and waveguides.

The patterned thick layer 32 can, e.g., be a thick photonic bandgapstructure. Thick photonic bandgap structures provide more efficientoptical edge coupling than thinner photonic bandgap structures that canbe made by dry etching.

FIGS. 2A and 2B show two exemplary planar photonic bandgap structures30A, 30B. Cross-sectional views through the structures 30A and 30B arefaithfully represented in FIG. 2. The structures 30A and 30B include alayer 32 of a metal-polar group III-nitride, i.e., (0 0 0 1)-plane groupIII-nitride. The layer 32 is located on the top surface of thecrystalline substrate 12 shown in FIG. 2. The layer 32 is mechanicallypatterned by an array of substantially identical columnar features 34A,34B. The columnar features 34A, 34B are holes and trenches in thestructures 30A and 30B, respectively.

The holes 34A and trenches 34B form regular arrays that have one and twodiscrete lattice symmetries, respectively. For this reason, the holes34A and trenches 34B produce respective 2-dimensional and 1-dimensionalperiodic modulations of the refractive index of the layer 32. Therefractive index modulations produce a photonic bandgap structure forselected lattice lengths in the arrays. Lattice lengths that are oddintegral multiples of ¼ times the effective wavelength of input light inthe medium will produce photonic bandgap structures.

FIG. 2C shows a photonic bandgap structure 30C similar to the photonicbandgap structure 30A of FIG. 2A except that the holes and the groupIII-nitride material layer are exchanged. In the structure 30C, thelayer 32C of group III-nitride is a two-dimensional array of isolatedpillars. Between the group III-nitride pillars is an interconnectedtwo-dimensional pattern of trenches 34C. The trenches 34C isolate thepillars from each other.

FIG. 3 illustrates a method 40 for fabricating a structure with amechanically patterned layer of group III-nitride, e.g., as shown inFIG. 1A-1B, 2, 2A, or 2B.

The method 40 includes forming a metal-polarity first layer of a firstgroup III-nitride on a selected planar surface of a crystallinesubstrate (step 42). Forming the layer includes performing an epitaxialgrowth of a first group III-nitride, and mechanically patterning thelayer lithographically. The composition of the first group III-nitrideis selected to insure that the epitaxial growth produces ametal-polarity layer. The mechanical patterning produces a regularpattern of identical holes or trenches that expose a portion of thesubstrate through the layer.

Next, the method 40 includes epitaxially growing a thicker second layerof a second group III-nitride over the first layer and the exposedportion of the substrate (step 44). Over the first layer, the secondlayer grows with metal-polarity. Over the exposed portion of thesubstrate, the second layer grows with N-polarity. The first and secondgroup III-nitrides have different alloy compositions, e.g., AlN and GaN,and have a wide lattice-mismatch with the substrate.

Finally, the method 40 also includes subjecting the second layer to anaqueous solution of a strong base such as potassium hydroxide (KOH) orsodium hydroxide (NaOH) (step 46). The aqueous solution mechanicallypatterns the second layer by selectively etching N-polar surfaces.Aqueous solutions of strong bases do not significantly etch metal-polarsurfaces of group III-nitrides. The form of the mechanical patterningqualitatively depends on the etching time and the concentration of theetchant.

FIG. 4 is a scanning electron micrograph (SEM) of a polarity striped GaNlayer 50 that has been wet etched with a 2 molar aqueous solution of KOHfor 45 minutes. The GaN layer was maintained at a temperature of about90° C. during the wet etch.

The etched GaN layer 50 has N-polar GaN stripes 52 and Ga-polar GaNstripes 54. The relatively short etch has removed significant materialfrom the N-polar GaN stripes 52 without removing significant materialfrom the Ga-polar GaN stripes 54. In the N-polar stripes 52, the etchproduces a surface formed of densely packed hexagonal GaN pyramids,e.g., as shown in FIG. 1B. The pyramids have various sizes and sharpapical tips with diameters of about 20-30 nm or less.

Measurements indicate that the pyramid density, ρ_(Δ), varies withetching temperature, T, as: [ρ_(Δ)]⁻¹=[ρ_(Δ0)]⁻¹exp(−E_(a)/k_(B)T) wherek_(B) is Boltzmann's constant. For a 2 molar solution of KOH, a 15minute etch, and temperatures between 25° C. and 100° C., measurementsshow that the activation energy E_(a) equal to about 0.587 eV.

The inventors believe that the wet KOH etch produces a distribution ofpacked hexagonal GaN pyramids, in part, due to the Ga-polar GaN stripes54 that are not etched. In particular, the Ga-polar stripes laterallyconfine the N-polar stripes 52 so that the etchant attacks top surfaceof the N-polar stripes 52 rather than side surfaces thereof. The KOH wetetch produces a dense-packing of sharp tipped hexagonal GaN pyramidswhen the N-polar GaN stripes 52 have widths of about 7 microns (μm). Itis believed that a dense packing of hexagonal pyramids will also resultfrom a wet KOH etch of GaN surfaces in which N-polar GaN stripes havewidths of about 100 μm or less. It is not however, believed that a wetKOH etch of an unconfined planar surface N-polar GaN will produce adense packing of sharp tipped, hexagonal GaN pyramids.

FIG. 5 is an SEM image showing a polarity-striped GaN layer 50 that hasbeen wet etched with a 4 molar aqueous solution of KOH for 60 minutes.Again, the GaN layer was maintained at a temperature of about 90° C.during the wet etch.

The more intense etch has removed all material from the N-polar GaNstripes 52 except for isolated hexagonal GaN pyramids 56. The wet etchstopped on the underlying crystalline sapphire substrate. Thisintermediate length etch produces patterning like that of FIG. 1A, atleast, within individual N-polar GaN stripes 52. Within these regions,the hexagonal GaN pyramids 56 have a random distribution.

FIG. 6 is an SEM image of a polarity-striped GaN layer 50 that has beenetched with a 4 molar aqueous solution of KOH for more than 60 minutes.Again, the GaN layer is maintained at a temperature of 25° C.-125° C.and preferably of about 90° C. during the wet etch.

This longer etch has completely removed the original N-polar GaN stripes52. As a result, substantially vertical trenches separate the unetchedGa-polar stripes 54. The sidewalls of the Ga-polar stripes 54 are notcompletely vertical, because the wet etchant slowly attacks sidewalls ofGa-polar GaN layers. Aqueous solutions with higher concentrations of KOHthan 4 molar tend to erode exposed side and end surfaces of Ga-polarstripes 54. The resulting structure has a patterned Ga-polar layer ofgroup III-nitride like structures 30, 30A, and 30B of FIGS. 2, 2A, and2B.

FIG. 7 illustrates a method 60 for fabricating GaN structures that aremechanically patterned as in FIGS. 1A, 1B, 2, 2A, and 2B. The method 60includes preparing a planar sapphire growth substrate (step 62), growingand patterning a Ga-polarity aligning layer on the substrate (step 64),and epitaxially growing a polarity-patterned GaN layer over the aligninglayer (step 66). The method 60 also includes wet etching the GaN layerto produce mechanical patterning by selectively removing GaN in theN-phase regions (step 68).

In step 62, preparing the sapphire growth substrate includes cleaning a(0 0 0 1)-plane surface of a crystalline sapphire substrate. Thecleaning includes washing the surface for 1 minute in an aqueouscleaning solution. Mixing a first aqueous solution having about 96weight % H₂SO₄ with a second aqueous solution having about 30 weight %H₂O₂ produces the aqueous cleaning solution. During the mixing, about 10volume parts of the first solution are combined with one volume part ofthe second solution. The cleaning also includes rinsing the washedsurface with de-ionized water and then spin-drying the sapphire growthsubstrate.

In step 62, preparing the growth substrate also includes degassing thesapphire substrate in the buffer chamber of a molecular beam epitaxy(MBE) system at about 200° C. The degassing continues until the chamberpressure is below about 5×10⁻⁹ Torr. After the degassing, the sapphiresubstrate is transferred to the growth chamber of the plasma-assistedMBE system.

In step 64, growing and patterning a Ga-polarity aligning layer includesperforming an MBE growth of an AlN layer on the sapphire substrate(substep 64 a). To perform the MBE growth, the temperature of the growthchamber is raised at a rate of about 8° C. per minute to a finaltemperature of about 720° C. The sapphire substrate is maintained at auniform temperature with the aid of a 300 nm thick layer of titaniumdeposited on the substrate's back surface.

The MBE system grows the AlN layer to a thickness of about 20 nm to 30nm. This thin AlN layer is sufficiently thick to cover the entireexposed surface of the sapphire substrate. In the model 32P MolecularBeam Epitaxy system made by Riber Corporation of 133 boulevard National,Boite Postale 231, 92503 Rueil Malmaison France, the growth conditionsare: Al effusion cell temperature of about 1050° C., nitrogen flow rateof about 2 sccm, and RF power of about 500 watts (W).

In step 64, forming the patterned AlN layer 12 includes performing anMBE growth of about 50 nm of protective GaN on the already grown AlNlayer (substep 64 b). The GaN layer protects the underlying AlN fromoxidation during subsequent removal of the substrate from the MBE growthchamber. Growth conditions for the GaN layer are similar to those forthe MBE growth of the AlN layer except that the temperature is raised inthe Ga effusion cell rather than in the Al effusion cell. During thisgrowth, the Ga effusion cell has a temperature of about 1000° C. toabout 1020° C.

After cooling the sapphire substrate to about 200° C., the GaN/AlN layeris lithographically patterned with a regular array of windows thatexpose selected portions of the sapphire substrate (substep 64 c). Thepatterning step includes forming a photoresist mask on the GaN layer andthen, performing a conventional chlorine-based plasma etch to removeunmasked portions of the GaN/AlN layer. Exemplary conditions for theplasma etch are: RF source power of about 300-500 watts, source bias of100 volts to 200 volts, chlorine-argon flow rate of about 10-25 sccm(20% to 50% of the flow being argon), and a gas pressure of about 1millitorr to about 10 millitorr. The plasma etch produces a preselectedpattern of GaN topped AlN regions.

After the plasma etch, the sapphire substrate with a pattern of GaNtopped AlN regions is cleaned in an aqueous solution of HCl, rinsed inde-ionized water, and blown dry with nitrogen. This aqueous cleaningsolution includes between about 36.5 weight % HCl and about 48 weight %HCl. Then, the above-described steps are again used to reintroduce thesapphire substrate into the MBE system.

In step 66, epitaxially growing a GaN layer includes performing a plasmaenhanced MBE growth of a GaN layer to a thickness of about 2 μm or more.During the MBE growth, the system conditions, are: Ga effusion celltemperature of about 1000° C. to about 1020° C., nitrogen flow rate ofabout 2 sccm, and RF power of about 500 watts (W). During this growth,the GaN topped AlN regions initiate growth of Ga-polar GaN, and theexposed regions of the sapphire substrate 10 initiate growth of N-polarGaN.

In step 68, the anisotropic wet etching includes immersing the GaN layerand substrate in an aqueous solution of KOH. Exemplary wet etches use 1to 4 molar aqueous solutions of KOH and etch periods of about 15 minutesto 60 minutes at temperatures of 100° C. The concentration of KOH andetch time determines the qualitative form of the resulting mechanicalpatterning as illustrated in FIGS. 4-6. The wet etch selectively removesGaN with N-polarity. Nevertheless, wet etches with more basic aqueoussolutions than 4 molar KOH can erode end faces of Ga-polar portions ofthe original GaN layer.

From the disclosure, drawings, and claims, other embodiments of theinvention will be apparent to those skilled in the art.

1-12. (canceled)
 13. An apparatus, comprising: a crystalline substratewith a planar surface; and a plurality of pyramidal field-emitterslocated over a portion the surface; a layer of a first group III-nitridelocated on another portion of the surface, the field-emitters comprisinga second group III-nitride; a layer of the second group-III nitridesemiconductor being over the layer of a first group III-nitride andbeing free of pyramidal surface structures; and wherein the first andsecond group III-nitrides have different alloy compositions.
 14. Theapparatus of claim 13, wherein the first group III-nitride comprisesaluminum and the second group III-nitride comprises gallium.
 15. Theapparatus of claim 13, further comprising: a phosphor screen located toreceive electrons emitted by the pyramidal field-emitters; and aplurality of gate electrodes located between the layer of the secondgroup III-nitride and the phosphor screen.
 16. The apparatus of claim15, wherein the gate electrodes are mechanically supported by the layerof the second group III-nitride.
 17. An apparatus, comprising: acrystalline substrate having a planar surface; a mechanically patternedlayer of a first group III-nitride being located on the planar surface;and a layer of a second group III-nitride being located on themechanically patterned layer of a first group III-nitride, the layer ofa second group III-nitride having a thickness of about 2 μm or more andhaving a pattern of columnar holes or trenches therein; and wherein theholes or trenches form a regular array; and wherein the first and secondgroup III-nitrides have different alloy compositions.
 18. The apparatusof claim 17, wherein bottom surfaces of at least some of the holes ortrenches contact the planar surface.
 19. The apparatus of claim 18,wherein the layer of a second group III-nitride comprises a regulartwo-dimensional array of pillars.
 20. The apparatus of claim 17, whereinthe second group III-nitride comprises gallium and the first groupIII-nitride comprises aluminum.